Switch pipelining metadata action to hollow rlib (-Zno-codegen)

Replace the --rustc-quit-on-rmeta / .rmeta approach with Buck2-style
hollow rlibs: the RustcMetadata action runs rustc to completion with
-Zno-codegen, emitting a .rlib archive (named -hollow.rlib) that
contains only metadata and optimized MIR.

Key fixes:
- Main Rustc action uses full rlib --extern deps so the SVH embedded
  in the full rlib matches what downstream binaries expect (avoiding
  E0460 with non-deterministic proc macros).
- RUSTC_BOOTSTRAP=1 is set on both actions: it changes the crate hash,
  so inconsistent use would cause SVH mismatch even for deterministic
  crates.
- -Ldependency= points to the _hollow/ subdirectory so the full rlib
  and hollow rlib never appear in the same search path (avoids E0463).
- transitive_metadata_outputs are always included in the sandbox so
  rustc can resolve transitive -Ldependency= references.

Adds an SVH mismatch test that demonstrates the problem with
non-deterministic proc macros (uses a HashMap-based derive macro whose
output varies by OS-seeded random seed).

Author: walter-zeromatter

cargo/Cargo.lock

@@ -205,11 +205,15 @@ def _rust_library_common(ctx, crate_type):
         crate_type,
         disable_pipelining = getattr(ctx.attr, "disable_pipelining", False),
     ):
+        # The hollow rlib uses .rlib extension (not .rmeta) so rustc reads it as an
+        # rlib archive containing lib.rmeta with optimized MIR. It is placed in a
+        # "_hollow/" subdirectory so the full rlib and hollow rlib never appear in the
+        # same -Ldependency= search directory (which would cause E0463).
         rust_metadata = ctx.actions.declare_file(
-            paths.replace_extension(rust_lib_name, ".rmeta"),
-            sibling = rust_lib,
+            "_hollow/" + rust_lib_name[:-len(".rlib")] + "-hollow.rlib",
         )
         rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)
+
         metadata_supports_pipelining = (
             can_use_metadata_for_pipelining(toolchain, crate_type) and
             not ctx.attr.disable_pipelining